David Notkin Autumn 2009 CSE303 Lecture 10 "If it weren't for C, we'd be writing programs in BASI, PASAL, and OBOL."

Today Some C leftovers from Monday C memory model; stack allocation –computer memory and addressing –stack vs. heap –pointers –parameter passing by value by reference CSE303 Au092

Arrays as parameters Arrays do not know their own size; they are just memory chunks – harder than in Java int sumAll(int a[]); int main(void) { int numbers[5] = {7, 4, 3, 15, 2}; int sum = sumAll(numbers); return 0; } int sumAll(int a[]) { int i, sum = 0; for (i = 0; i <... ??? }

Solution 1: declare size Declare a function with the array's exact size int sumAll(int a[5]); int main(void) { int numbers[5] = {7, 4, 3, 15, 2}; int sum = sumAll(numbers); return 0; } int sumAll(int a[5]) { int i, sum = 0; for (i = 0; i < 5; i++) { sum += i; } return sum; }

Solution 2: pass size Pass the array's size as a parameter int sumAll(int a[], int size); int main(void) { int numbers[5] = {7, 4, 3, 15, 2}; int sum = sumAll(numbers, 5); return 0; } int sumAll(int a[], int size) { int i, sum = 0; for (i = 0; i < size; i++) { sum += i; } return sum; }

Returning an array arrays (so far) disappear at the end of the function: this means they cannot be safely returned int[] copy(int a[], int size); int main(void) { int numbers[5] = {7, 4, 3, 15, 2}; int numbers2[5] = copy(numbers, 5); // no return 0; } int[] copy(int a[], int size) { int i; int a2[size]; for (i = 0; i < size; i++) { a2[i] = a[i]; } return a2; // no }

Solution: output parameter workaround: create the return array outside and pass it in -- "output parameter" works because arrays are passed by reference void copy(int a[], int a2[], int size); int main(void) { int numbers[5] = {7, 4, 3, 15, 2}; int numbers2[5]; copy(numbers, numbers2, 5); return 0; } void copy(int a[], int a2[], int size) { int i; for (i = 0; i < size; i++) { a2[i] = a[i]; }

A bit about strings (more soon) String literals are the same as in Java –printf("Hello, world!\n"); –but there is not actually a String type in C; they are just char[] Strings cannot be made, concatenated, or examined as in Java: String s = "hello"; // no int answer = 42; printf("The answer is " + answer); // no int len = "hello".length(); // no int printMessage(String s, int times) {... // no

Memory hierarchy CPU registers L1/L2 cache (on CPU) physical RAM (memory) a few bytes 1-4 MB virtual RAM (on a hard disk) secondary/permanent storage (hard disks, removable drives, network) 1-2 GB 2-8 GB 500 GB

Virtual addressing each process has its own virtual address space of memory to use –each process doesn't have to worry about memory used by others –OS maps from each process's virtual addresses to physical addresses reality process 1 process 2 virtual

Process memory layout when a process runs, its instructions/globals load into memory address space is like a huge array of bytes –total: 2 32 bytes –each int = 4 bytes as functions are called, data goes on a stack dynamic data is created on a heap stack (function calls) available memory heap (dynamically allocated data) global/static variables ("data segment") code instructions ("text segment") 0x xFFFFFFFF address space

Stack frames stack frame or activation record: memory for a function call –stores parameters, local variables, and return address to go back to int f(int p1, int p2) { int x; int a[3];... return x + y; } stack available heap global data code offsetcontents 24 p2 20 p1 16return address 12 a[2] 8 a[1] 4 a[0] 0 x

gpgp f p1, p2 x, a Tracing function calls int main(void) { int n1 = f(3, -5); n1 = g(n1); } int f(int p1, int p2) { int x; int a[3];... x = g(a[2]); return x + y; } int g(int param) { return param * 2; } stack available heap global data code main f g main n1 gpgp

The & operator &variable produces variable 's memory address #include int main(void) { int x, y; int a[2]; // printf("x is at %d\n", &x); printf("x is at %p\n", &x); // x is at 0x0022ff8c printf("y is at %p\n", &y); // y is at 0x0022ff88 printf("a[0] is at %p\n", &a[0]); // a[0] is at 0x0022ff80 printf("a[1] is at %p\n", &a[1]); // a[1] is at 0x0022ff84 return 0; } –%p placeholder in printf prints a memory address in hexadecimal

Danger! array bounds are not enforced; can overwrite other variables #include int main(void) { int x = 10, y = 20; int a[2] = {30, 40}; printf("x = %d, y = %d\n", x, y); // x = 10, y = 20 a[2] = 999; // !!! a[3] = 111; // !!! printf("x = %d, y = %d\n", x, y); // x = 111, y = 999 return 0; }

Segfault segmentation fault ("segfault"): A program crash caused by an attempt to access an illegal area of memory #include int main(void) { int a[2]; a[999999] = 12345; //out of bounds return 0; }

Segfault #include void f() { f(); } int main(void) { f(); return 0; } CSE303 Au0917

The sizeof operator sizeof(type) or (variable) returns memory size in bytes #include int main(void) { int x; int a[5]; printf("int=%d, double=%d\n", sizeof(int), sizeof(double)); printf("x uses %d bytes\n", sizeof(x)); printf("a uses %d bytes\n", sizeof(a)); printf("a[0] uses %d bytes\n", sizeof(a[0])); return 0; } Output: int=4, double=8 x uses 4 bytes a uses 20 bytes a[0] uses 4 bytes

sizeof continued arrays passed as parameters do not remember their size #include void f(int a[]); int main(void) { int a[5]; printf("a uses %d bytes\n", sizeof(a)); f(a); return 0; } void f(int a[]) { printf("a uses %2d bytes in f\n", sizeof(a)); } Output: a uses 20 bytes a uses 4 bytes in f

Pointer: a memory address referring to another value type* name; // declare type* name = address; // declare/initialize int x = 42; int* p; p = &x; // p stores address of x printf("x is %d\n", x); // x is 42 printf("&x is %p\n", &x); // &x is 0x0022ff8c printf("p is %p\n", p); // p is 0x0022ff8c int* p1, p2; // int* p1; int p2; int* p1, *p2; // int* p1; int* p2

Dereferencing: access the memory referred to by a pointer *pointer // dereference *pointer = value; // dereference/assign int x = 42; int* p; p = &x; // p stores address of x *p = 99; // go to the int p refers to; set to 99 printf("x is %d\n", x); Output: x is 99

* vs. & many students get * and & mixed up –& references (ampersand gets an address) –* dereferences(star follows a pointer) int x = 42; int* y = &x; printf("x is %d \n", x); // x is 42 printf("&x is %p\n", &x); // &x is 0x0022ff8c printf("y is %p\n", y); // y is 0x0022ff8c printf("*y is %d \n", *y); // *y is 42 printf("&y is %p\n", &y); // &y is 0x0022ff88 What is *x ?

L-values and R-values L-value: Suitable for being on left-side of an = assignment -- a valid memory address to store into R-value: Suitable for right-side of an = assignment int x = 42; int* p = &x; L-values : x or *p (store into x ), p (changes what p points to) not &x, &p, *x, *(*p), *12 R-values : x or *p, &x or p, &p not &(&p), &42

Pass-by-value: copy parameters' values Cannot change the original (“actual”) parameter variable int main(void) { int a = 42, b = -7; swap(a, b); printf("a = %d, b = %d\n", a, b); return 0; } void swap(int a, int b) { int temp = a; a = b; b = temp; }

Pass-by-reference: point to parameters Can change the actual parameter variable using the “formal” int main(void) { int a = 42, b = -7; swap(a, b); printf("a = %d, b = %d\n", a, b); return 0; } void swap(int a, int b) { int temp = a; a = b; b = temp; }

Questions? CSE303 Au0926